This invention relates to dampening roll covers for use in lithographic press rolls, and more particularly it relates to knitted dampening roll covers which are composed of elastic yarns and bulked filament yarns to provide a close fitting seamless cover which does not wrinkle, does not have to be adhesively secured to the core and does not leave the pattern of the cover in the printed copy.
It has been known in the art to provide dampening roll covers of knapped flannel, of the type known as molleton, in the form of sewn tubular sleeves as well as in the form of a spiral wrap of slit flannel adhesively secured to the roll surface. It has also been proposed to provide dampening roll covers of a plain tubular knit construction, or terrycloth construction, in sleeve form. Although such knitted sleeves may possess some stretch, they are not, in general, truly form fitting, and must be manufactured to such close tolerances that their application to a rubber dampening roll becomes a major effort, involving the use of slip-sheets of acetate film to serve as a friction reducing layer between the rubber and the knitted sleeve. Removal is somewhat laborious, since prior art sleeves must be cut from the roll, with the danger of damage to the underlying rubber surface. Additionally, since prior art sleeves have no elastic recovery they must be anchored on the roll, which is customarily done by means of draw strings in either end of the sleeve or cover.
In my earlier U.S. Pat. No. 3,180,115, it was found that by the incorporation into a dampening roll sleeve of elastic yarns, by which was meant true rubber yarns, spandex yarns, and similar elastomeric yarns with essentially complete and immediate recovery from deformation within elongations of from 10% to 300%, a superior tubular dampening roll sleeve is provided which eliminates many of the disadvantages inherent in prior art covers. The dampening roll cover of that invention was superior to prior art covers in a number of respects but suffered from a major drawback -- it frequently produced a pattern in the printed copy. The wrapped yarn used therein as the inelastic surface yarn provided an essentially compact central element with protruding loops that radiated from that central element of the yarn, but these loops made very little contribution to the bulk and softness of the central yarn core element that formed the knitting stitch. Accordingly, the knitting stitch was not deformed by the pressure of nip into a smooth surface, but maintained, rather, because of its relatively incompressible nature, a surface pattern that showed the shape of the stitch and transmitted this non-uniformity in the form of a pattern of moisture onto the surface of the plate, thereby producing a pattern in the printed copy.
Accordingly, it is an object of this invention to provide a dampening roll cover for use in lithographic press rolls which has all the advantages of the elastic, knitted dampening roll cover described above but does not suffer from the major drawback therein of leaving a pattern in the printed copy produced thereby.
It is another object of this invention to provide a form fitting elastic dampening roll cover comprising elastomeric yarns together with a bulked filament yarn which will have the bulk and softness that will be deformed by the nip pressure so as to produce a more evenly formed surface therein that will reduce patterns in the printed copy.
Other objects of the invention will be apparent from the description below and from the example contained therein.